The invention relates generally to an improvement in the design of an oil and gas well perforating device. The improvement applies to a type of perforating device that is typically lowered into the well through the casing or tubing in the well to a position where the explosive charges are detonated at the desired depth. The improvement is a method of decreasing the distortion of the device after detonation to ensure that it can be pulled back out of the well.
After an oil or gas well is drilled, steel casing is lowered into the well and cemented to the adjoining rock formations. Typically, perforations are needed to allow the oil or gas from the desired rock formation to be able to flow into the casing and then out of the well. The perforations are made by lowering, on a wireline or tubing, the perforating gun containing explosive charges to the desired depth and detonating the charges. There are several different types of perforating guns.
One type of perforating gun is referred to as a casing gun. A casing gun is a hollow steel carrier that is lowered into the casing of the well with the perforations made through screwed in ports. This type of perforating gun is of sufficient size that there is not usually significant distortion caused to the carrier from the explosive charge and the hollow steel carrier can be reused a number of times. There is not any issue as to the removal of this type of perforating gun after the detonation as there is not any distortion and the clearances between the casing and the perforating gun are more than sufficient.
A second type of perforating gun is an expendable casing gun. This is similar to the previously discussed casing gun with the addition of larger charges that will cause significant distortion to the hollow steel carrier. The distortion is sufficient to make the hollow steel carrier useable only one time and therefore expendable. The larger charges are sometimes needed when greater penetration is required such as when some of the rock formation has washed away and there is a greater amount of cement to penetrate. The distortion can be sufficient as to cause retrieval problems.
A third type of perforating gun is a tubing conveyed perforating gun. The tubing is a retrievable string of pipe inside of the casing that is permanently cemented in place. This is another type of casing gun except the carrier is made a part of the tubing string rather than being run on the wireline. Depending on the size of the charge, distortion of the carrier can be sufficient to make the perforating gun expendable. The distortion can be sufficient as to cause retrieval problems.
All of the previously discussed perforating guns are made to be lowered into the casing. There are also perforating guns made to be lowered into the tubing. These through tubing perforating guns are designed to be utilized while leaving the tubing inside the well and casing. In order for the perforating guns to be lowered inside of the tubing requires a smaller diameter perforating gun. The through tubing perforating guns are lowered through the tubing to a desired depth, below the bottom of the tubing, at the desired rock formation.
A fourth type of perforating gun is a through tubing strip gun run on wireline. This type of perforating gun includes a strip carrier on which capsule shaped charges may be mounted. The capsule shaped charges are sealed to protect the charges from the well environment. At detonation the strip gun is basically blown apart and the debris drops to the bottom of the well below the perforations. Any intact portion of the strip gun is then retrieved through the tubing.
A fifth type of perforating gun is the retrievable through tubing gun which is like the casing gun in that it uses a sealed carrier to hold the charges but is a smaller diameter to fit inside the tubing. The smaller diameter is not as capable of absorbing the explosive charge and the outer diameter of the hollow carrier is distorted. The distortion occurs at the level of the charge around most of the circumference of the sealed hollow carrier. This distortion can be enough to prevent the perforating gun from being retrieved back into the tubing after being fired.
All of the perforating guns discussed utilize a sealed carrier with the exception of the strip gun. The sealed carriers have recessed areas at the location of the charges. The recessed area is to reduce the amount of energy the charge loses in exiting the perforating gun.
The swelling and distortion of the perforating gun, caused by the explosive charge, is an important consideration when using the expendable casing gun, the tubing conveyed perforating gun and the retrievable tubing gun. There has been a patented well perforating device of Dobrinski, U.S. Pat. No. 4,919,050, that was designed for the purpose of reducing the excessive deformation. That patent uses a band of removed steel cut all the way around the hollow carrier, or body member, such that the outward distortion does not exceed the outer diameter of the perforating gun. The present invention offers many advantages over the Dobrinski patent. The present invention removes much less steel as the faceted cuts absorb energy, and the cuts are only made where they are necessary. This structurally stronger design of the present invention allows for achieving the primary objective of limiting the outward distortion to the outer diameter of the body member while at the same time achieving all other necessary aspects of a perforating gun, and providing additional benefits such as being made out of standard steel.
There continues to be a need to be able to minimize the outward distortion of some perforating guns. Excessive distortion can result costly remedial measures and can even run the risk of making the well junked and abandoned. The method of reducing the distortion must also maintain the performance of the perforating charges and maintain sufficient integrity of the carrier housing.